Code:
Weapon                                 Weight
ak47                                    2.00
aug                                     2.00
awp                                     0.50
deagle                                  3.00
elite                                   3.01
famas                                   2.25            
fiveseven                               3.10
g3sg1                                   0.50
galil                                   2.25            
glock                                   3.50
hegrenade                               4.00
knife                                  10.00
m249                                    1.25            
m3                                      1.75
m4a1                                    2.00            
mac10                                   2.75            
mp5navy                                 2.50            
p228                                    3.10
p90                                     2.30                                
scout                                   3.75
sg550                                   0.50
sg552                                   2.00            
tmp                                     2.75            
ump45                                   2.60
usp                                     3.10
xm1014                                  1.75
zombie_claws_of_death                   1.10
The weight is the multiple that will be applied to skill changes when a kill or zombie infection occurs. So a kill with a run with 2.00 weight is worth twice the skill change then a weapon with 1.00. This effects both the amount you gain and the amount your victim loses.